That is a great question,...and one that opens up a lot of other questions about what factors in making a call have the most influence on how a call will sound.
In answer to your specific question, I use the "eyeball" method,...and that is primarily because I don't particularly think that being very exacting on reed spacing is going to insure that one call will sound like another. There are just too many variables in the process of making mouth calls,....and from what I have seen in my experimenting, very minute differences in any of those variables can have a tremendous impact on how a call will sound.
Probably the primary limiting factor in call consistency is the latex material itself. To assume that the sound producing qualities are the same for any two pieces of latex material cut out of a sheet that is made for an application far removed from making turkey calls is a mighty big stretch, in my opinion. When you combine multiple layers of that material, space it, stretch it, and make cuts in it, the chances of producing a call that will sound exactly the same time after time is pretty slim,....and that is even if you don't take into consideration the various difference in the mechanics and abilities of the person that ends up with that call.
In summary, I do not believe that someone taking the time to try to space the reeds of a call to some minute tolerances, when taking into account all of the other factors involved, is going to make a lot of difference when all is said and done.
